Process for producing titanium oxide thin film, and photocatalyst

ABSTRACT

The present invention relates to a process for producing a titanium oxide thin film, said process comprises soaking a substrate in an aqueous solution containing 10 -9  to 9×10 -2  mol/L of a titanium fluoro complex compound in the presence of a fluoride ion capturing agent to form a titanium oxide thin film on the surface of the substrate; and a photocatalyst comprising the titanium oxide thin film thus produced; and can provide a simple process for the formation of a highly homogeneous titanium oxide thin film which is fast and corrosion resistant, and has a high refractive index and a catalytic activity for the photoreaction.

TECHNICAL FIELD

The present invention relates to a process for forming a titanium oxidethin film on the surface of a substrate and more specifically to aprocess for forming a titanium oxide thin film using the liquid phaseprecipitation process. The present invention also relates to aphotocatalyst comprising a titanium oxide thin film thus produced.

BACKGROUND ART

Titanium oxide produces an OH radical having a large energycorresponding to 120 kcal/mol by irradiation of light. This energy islarger than the bond energy of a C--C bond, a C--H bond, a C--N bond, aC--O bond, an O--H bond, or an N--H bond of organic compounds and thusthe energy can easily decompose these bonds. Because of this action,titanium oxide can easily decompose organic compounds, and therefore ithas been used as a catalyst of photodecomposition reactions to decomposea variety of organic substances including the decomposition,sterilization etc. of harmful substances or bad-smelling substancesdissolved in water or suspended in the air, thereby finding practicalapplications in environmental clarification, epidemics control, and thelike.

Such a utilization of titanium oxides as a catalyst can be effectivelycarried out by forming titanium oxide thin films on the surface of asubstrate including ceramics such as glass and tiles or inorganicfibers. On the other hand, by utilizing properties that the titaniumoxide thin films have high refractive indices and are chemically stable,titanium oxide thin films formed on the surface of light-transmittingsubstances such as glass have been used for optical applications such ascoating of optical lenses and for heat radiation reflexive glasses, andare expected to be a solar cell which makes possible low-cost solarpower generation.

As the processes for forming a titanium oxide thin film on the surfaceof a substrate, there are mentioned CVD, ion plating, sputtering, andthe like. In these processes, however, special and expensive equipmentis required, and the formation of a thin film on the substrate having alarge surface area and the formation of a homogeneous thin film on thesurface of a substrate having a complicated shape are difficult.

Furthermore, there are a process for forming titanium oxide thin filmsby spraying onto the surface of a substrate a solution in which atitanium alkoxide such as tetraethyl titanate, tetra-n-propyl titanate,tetraisopropyl titanate, or tetra-n-butyl titanate; or a titaniumchelate compound such as titanium acetyl acetonate is dissolved in anorganic solvent, or by soaking a substrate in said solution, removingthe solvent and then by oxidizing at a high temperature, or a processfor exposing the substrate to the vapor of titanium tetrachloride.However, in addition to being complicated, these processes requiredealing with hydrolytic substances and so it is impossible to form auniform thin film on the substrate having a large surface area or thesubstrate having a complicated shape.

Another process for forming titanium oxide thin films on the surface ofa substrate involves spreading a mixture obtained by kneading finelypowdered titanium oxide with a binder and a dispersant on the surface ofa substrate followed by drying thereof. However, in such a process, itis difficult to form a sufficiently thin and uniform as well as a strongthin film. Furthermore, due to the photocatalytic activity possessed bytitanium oxide, the binder which is an organic substance is decomposed,and therefore adhesiveness onto the substrate cannot permanentlycontinue.

In Japanese Unexamined Patent Publication No. 59-141441 and JapaneseUnexamined Patent Publication No. 1-93443, processes for forming atitanium oxide-coated layer on the surface of the substrate by soaking asubstrate in an aqueous solution containing fluorotitanate in thepresence of a fluoride ion-capturing agent such as boric acid have beendisclosed. In the former, zinc oxide is saturated in said aqueoussolution so that there is a problem of causing variation in refractiveindex due to introduction of Zn atom into the coated layer. Theconcentration of fluorotitanic acid is 0.3 to 1.0 mol/L in the formerand 0.1 to 3 mol/L in the latter, and a fluorotitanic acid solution of0.5 to 3.4 mol/L has been used in the Example therein. When such a highconcentration of fluorotitanic acid is used, the solution becomes turbidwith the addition of a fluoride ion-capturing agent since a largequantity of titanium oxide precipitates in the solution, which preventsformation of a thin film on the surface of the substrate without wastingthe materials. Furthermore, the formed thin film is ununiform and thesurface condition thereof is bad so that it is not suitable for opticalapplications in which its transmittance to light and its uniformity arerequired and for photoreactive catalysts or solar cells.

Thus, an object of the present invention is to provide a process forforming a titanium oxide thin film excellent in uniformity on thesurface of a substrate by means of simple equipment by overcoming theabovementioned problems associated with the conventional processes.Another object of the present invention is to provide a catalyst forphotodecomposition reaction, said catalyst comprising a titanium oxidethin film formed on the surface of the substrate.

SUMMARY OF THE INVENTION

The present inventors have conducted intensive studies to attain theabove-mentioned objects, and as the results, they have found that saidobjects can be attained by limiting the concentration of the titaniumfluoro complex compound in the aqueous solution used to precipitate thetitanium oxide thin film to a dilute range of 10⁻⁹ to 9×10⁻² mol/L andthereby have arrived at the present invention.

Thus, the process of producing a titanium oxide thin film of the presentinvention comprises soaking a substrate in an aqueous solutioncontaining 10⁻⁹ to 9×10⁻² mol/L of one kind or two or more kinds oftitanium fluoro complex compound in the presence of a fluorideion-capturing agent to form a titanium oxide thin film on the surface ofa substrate. The catalyst of the present invention also comprises thetitanium oxide thin film formed on the surface of the substrate by theabove-mentioned producing process, and is used in the photodecompositionreaction of organic compounds and nitrogen oxides, e.g., a trace amountof organic compounds.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an X-ray diffraction pattern of the titanium oxide filmobtained in Example 4.

FIG. 2 is a drawing showing a photocatalytic reaction of the titaniumoxide film formed on the surface of the soda lime glass obtained inExample 13 against an oil.

BEST MODE FOR PRACTICING THE INVENTION

In the present invention, the titanium fluoro complex compound used forthe formation of a titanium oxide thin film is a water-soluble acid or asalt which is represented by the general formula (I):

    A.sub.a Ti.sub.b F.sub.c                                   (I)

wherein A's may be the same or different from each other and eachrepresent a hydrogen atom, an alkali metal atom, an ammonium group orcoordinated water; and, a, b and c are each a number of making saidcomplex compound electrically neutral.

As A, there are mentioned a hydrogen atom; an alkali metal atom such aslithium, sodium, potassium, rubidium, and cesium; and an ammonium groupand coordinated water. In the above formula, when b is 1, a is usually2, c is usually 6. That is, while said complex compound is typicallyrepresented by A₂ TiF₆, a polynuclear complex compound having a pluralnumber of Ti atoms can be obtained. As the titanium fluoro complexcompounds represented by the general formula (I), there are exemplifiedby H₂ TiF₆, (NH₄)₂ TiF₆, Na₂ TiF₆, K₂ TiF₆, Rb₂ TiF₆, Cs₂ TiF₆, and thelike.

The titanium fluoro complex compound to be used in the present inventionmay be any material which is produced by any process. For example,powdered titanium oxide may be treated with hydrofluoric acid to makefluorotitanic acid. As the titanium oxide, any of the rutile structure,the anatase structure, the brookite structure, and amorphous may beused.

Alternatively, a hydroxide or an oxyhydroxide of titanium may bedissolved in an aqueous solution of an alkali metal hydrogen difluoridesuch as ammonium hydrogen difluoride or sodium hydrogen difluoride tosynthesize a titanium fluoro complex compound, and it may be used in theprocess for production of the present invention.

The titanium fluoro complex compounds may be used as aqueous solutionsin the concentration of 10⁻⁹ to 9×10⁻² mol/L, preferably 10⁻⁶ to 6×10⁻²mol/L, and more preferably 10⁻² to 4×10⁻² mol/L. For example, 0.001 to20 g or so of a titanium oxide is dissolved in 400 ml of a 0.001 to 0.6mol/L hydrofluoric acid aqueous solution to obtain an aqueous solutioncontaining titanium fluoro complex ions of 10⁻⁹ to 9×10⁻² mol/L. Whenthe concentration is less than 10⁻⁹ mol/L, a thin film cannot be formedon the surface of the substrate, while if an aqueous solution exceeding9×10⁻² mol/L is used, the solution becomes turbid with the addition of afluoride ion-capturing agent or the solution thereof, whereby a uniformtitanium oxide thin film cannot be formed on the surface of thesubstrate. The "aqueous solution" herein mentioned may be an aqueoussolution containing an excess of hydrogen fluoride which is used tosynthesize the above complex compound from the titanium oxide describedabove. Also, an excessive amount of titanium oxide is further added tothe prepared titanium fluoro complex compound aqueous solution(hereinafter as defined above) to prepare a saturated solution of theabove-mentioned complex compound, then, titanium oxide which is notdissolved is removed by filtration and the resulting aqueous solutionmay be used. Incidentally, in the present invention, an aqueous solutionincludes those which contain an organic solvent soluble in water, and aslong as it comprises water as a main component, it is not prevented fromexisting, for example, an alcohol such as methanol, ethanol, etc.; anether such as dimethyl ether, diethyl ether, etc.; a ketone such asacetone, etc.; and other organic solvent soluble in water.

Furthermore, a seed crystal(s) for producing a titanium oxide thin filmmay be added to an aqueous solution of such a titanium fluoro complexcompound. The seed crystal(s) to be used is preferably a crystal of thedesired titanium oxide. The average particle size of the seed crystalmay be as small as 0.001 to 10 μm, and the amount added may be, thoughnot limited, a minute amount. With the addition of the seed crystal(s),the precipitate formation rate of the crystalline titanium oxide thinfilm can be enhanced.

The fluoride ion-capturing agent to be used in the present inventionincludes a homogeneous type which is used by being dissolved in a liquidphase and a heterogeneous type which is a solid material. Depending onthe purposes, one of them may be used, or both of them may be used incombination.

The homogeneous type fluoride ion-capturing agent reacts with a fluorideion to form a stable fluoro complex compound and/or a stable fluoride,thereby shifting the equilibrium of the hydrolysis reaction so as toprecipitate a titanium oxide thin film on the surface of a substrate.Examples thereof may include in addition to boron compounds such asorthoboric acid, metaboric acid, boron oxide, etc.; aluminum chloride,sodium hydroxide, aqueous ammonia and the like. For example, when TiO₂is to be precipitated from H_(a) Ti_(b) F_(c) (wherein a to c are asdefined above) by using orthoboric acid, and, for example, when H₂ TiF₆is mentioned as an example, the reaction represented by the formula(III) is shifted in the direction of consuming F⁻ so that theequilibrium represented by the formula (II) is shifted in the directionof forming F⁻, and as the results, a thin film comprising TiO₂ isprecipitated. Such a capturing agent is generally used in the form of anaqueous solution, but it may be added in the form of a powder, which isthen dissolved in the system. Addition of said capturing agent may becarried out at one time or intermittently in several divided amounts, orit may be carried out continuously at a controlled supplying rate, forexample, at a constant rate.

    TiF.sub.6.sup.2- +2H.sub.2 O⃡TiO.sub.2 +6F.sup.- +4H.sup.+(II)

    BO.sub.3.sup.3- +4F.sup.- +6H.sup.+ →BF.sub.4.sup.- +3H.sub.2 O(III)

The heterogeneous type fluoride ion-capturing agent may be exemplifiedby a metal such as aluminum, titanium, iron, nickel, magnesium, copper,zinc, germanium, etc.; ceramics such as glass; silicon; a boron compoundsuch as orthoboric acid, metaboric acid, boron oxide, etc.; and acompound such as calcium oxide, aluminum oxide, silicon dioxide,magnesium oxide, etc. When such a solid material is added or insertedinto an aqueous solution, F⁻ in the vicinity of the solid material isconsumed to cause a decrease in its concentration, with a result thatthe chemical equilibrium in the region is shifted to precipitatetitanium oxide. When such a solid material is used, depending on theprocess of insertion and the reaction condition, the titanium oxide thinfilm can be precipitated on the entire surface of the substrate soakedin the aqueous solution, or precipitation can be limited to a selectedlocal portion, i.e., the vicinity where said solid material exists.Whereas the main object of the present invention is to obtain a uniformtitanium oxide thin film, depending on the objects, a precipitated thinfilm on the surface of the substrate can be thickened partially by usingthe homogeneous- and heterogeneous-type fluoride ion-capturing agents incombination.

The amount of the homogeneous-type fluoride ion-capturing agent may varydepending on the kind and shape of the precipitates, but it is generallyin the range of 10⁻⁴ to 3,000% and preferably 10⁻¹ to 1,000% based onthe amount corresponding to the equivalent amount of fluoride ions inthe solution. The heterogeneous type fluoride ion-capturing agent is notparticularly limited and is preferably used in an amount in which theobjects and effects of the present invention are accomplished.

As the substrate, there may be used a wide range of substances whichcarries titanium oxide thin film to be formed, or which is coated forthe specific objects by the formed said thin film, or for beingprotected from oxidation, etc. Such a substance may be exemplified by ametal, ceramics, an organic polymer material, etc.

Especially, when the titanium oxide thin film is used as a catalyst forthe photodecomposition reaction, as a solar energy-absorbing layer of asolar cell, or as a coating layer for an optical purpose, a substancehaving high transparency such as glass, a polycarbonate, an acrylicresin, etc. is used as the substrate. When glass is used as thesubstrate, the kind is not limited. Particularly, even when a film isformed on an alkali-containing glass such as soda lime glass, there is alittle or no fear of deteriorating the film by dissolving the alkalicomponent into the film since the film is formed at a relatively lowtemperature in the process of the present invention.

The timing of soaking the substrate in the aqueous solution of thetitanium fluoro complex compound may be before, simultaneously with, orafter addition or insertion of the fluoride capturing agent. However,when a substrate which may be corroded by the system is used, it isnecessary to pay attention to the composition of the solution, thereaction conditions and the timing of soaking. The substrate may be ofany shape, is not restricted to the plate form, and those of complicatedshapes may be used. Also, in order to enhance uniformity of the film,for example, the substrate may be slowly rotated at a rotating rate of10 rpm or less, preferably 5 rpm or less.

The reaction temperature affects precipitation of the film so that it isusually set in the range of 10 to 80° C., preferably 20 to 50° C., andmore preferably 35 to 40° C. The reaction time is also optional, and,for example, when the desired precipitation is a much amount, thereaction time is elongated depending thereon. For example, in order toprecipitate a film having a thickness of about 0.2 μm, the reaction timeis preferably 1.5 to 24 hours, and more preferably 3 to 10 hours. Whenthe reaction time is shorter than the above, the film is notprecipitated sufficiently, while when it is longer than the above, thereis a fear that the film may peel off.

Thus, a uniform and fast titanium oxide thin film can be formed on thesurface of the substrate. The thin film thus formed is obtained as acrystallized titanium oxide thin film without undergoing the heatingprocess such as firing by optionally setting the precipitationconditions, but the heating process may be included depending on thepurpose. The heating process may be conducted, for example, at 200 to600° C. for about 0.5 to 5 hours. The titanium oxide thin film thusformed on the surface of the substrate has a thickness of 0.1 to 5.0 μmand a pencil hardness of 6 H to 7 H or more.

Industrial Applicability

In accordance with the present invention, a titanium oxide thin film canbe easily formed on the surface of a substrate, particularly on thesurface of the substrate having a large area or a complicated shape. Thethin film thus obtained does not have any strain caused by a coolingprocess since it does not necessarily require a heating process tocrystallize the thin film.

The titanium oxide thin films obtained in accordance with the presentinvention are fast and corrosion-resistant, and have a high refractiveindex, and a catalytic activity to the photoreaction. Utilizing thesevarious properties, they are extremely useful as a photocatalyst fordecomposition of an organic compound or a nitrogen oxide; as a solarcell; and for optical purposes such as coating of optical lenses, etc.When it is used as a catalyst for the photodecomposition reaction, aheating process is not particularly required so that the subject of amaterial of the substrate is a wide range whereby it can be easilyemployed in-place application such as a purification system forhousehold use and the waste water treatment at plants, etc.

EXAMPLES

In the following, the pre sent invention is explained in more details byreferring to Examples. The present invention is not limited by theseExamples.

Example 1

To 500 ml of purified water were added 2 ml of 46% hydrofluoric acid and7 g of the rutile structure titanium oxide powder, and the mixture wasthen dissolved while stirring at 35° C. for 24 hours. The powder oftitanium oxide remained without dissolution was removed by filtrationusing a filter having pores sized 11 μm. The concentration of thefluorotitanic acid solution obtained was 10⁻³ mol/L and a minute amountof titanium oxide sized a several μgm that passed through the filter waspresent as seed crystals. A glass substrate that had been previouslywashed with acetone was soaked in the solution at a temperature of 35°C., and each 5 g of orthoboric acid was added for 6 times with aninterval of one hour. When the obtained mixture was allowed to stand forfurther 6 hours, a uniform and transparent thin film having nointerference color was formed on the surface of the substrate while theliquid remained transparent.

The substrate was picked up from the liquid, washed with water anddried. The thin film on the substrate was dense and fast, and no scratchwas observed. When the film was subjected to the energy dispersive X-rayanalysis (EDX), it was observed that the crystals containing Ti wereformed, thereby confirming that the thin film was titanium oxide.

Example 2

In the similar manner as in Example 1 except for using 5 g of the rutilestructure titanium oxide powder, a fluorotitanic acid solution with aconcentration of 4×10⁻⁴ mol/L and containing a minute amount of seedcrystals was obtained. To the solution was added each 2 g of orthoboricacid for 5 times with an interval of one hour. After allowing theobtained mixture to stand for 12 hours, the orthoboric acid was added inthe similar manner for 6 more times and the mixture was allowed to standfor 3 days, a uniform and fast thin film was formed on the surface ofthe substrate. In the similar manner as in Example 1, it was confirmedthat the thin film was titanium oxide.

Example 3

In the similar manner as in Example 1 except for changing the addedamount of 46% hydrofluoric acid solution to 1 ml, a fluorotitanic acidsolution with a concentration of 10⁻⁴ mol/L and containing a minuteamount of seed crystals was obtained. To the solution was added each 5 gof orthoboric acid for 2 times with an interval of two hours. When themixture was allowed to stand for 5 days, a uniform and fast thin filmwas formed on the surface of the substrate. In the similar manner as inExample 1, it was confirmed that the thin film was titanium oxide.

Example 4

0.933 g of (NH₄)₂ TiF₆ was added to 400 ml of purified water, and wasdissolved by stirring at 30° C. for 24 hours to obtain an aqueoussolution of (NH₄)₂ TiF₆ with a concentration of 1.179×10⁻² mol/L. Each30 ml portion of this solution was taken in two polystyrene containers.Each glass plate which had been previously subjected to ultrasonicwashing in ethanol was soaked in each container as a substrate, and 1.06ml (1 equivalent) or 2.12 ml (2 equivalents) of an orthoboric acidaqueous solution with a concentration of 0.5 mol/L was added thereto,respectively, and the mixture was maintained at 30° C. for 6 days.

The substrates were taken out from the mixture, washed with distilledwater and air dried, and then observed by the scanning electronmicroscope (SEM). As a result, the formation of a dense thin film wasobserved. When the film was subjected to EDX, it was confirmed thatcrystals each containing Ti had been precipitated. Furthermore, as theresult of the X-ray diffraction (XRD), an XRD peak corresponding to theXRD peak of the anatase structure TiO₂ type powder was observed, therebyit could be confirmed that a thin film containing the anatase structureTiO₂ crystals had been formed. The result is shown as 1 in FIG. 1.However, the sample in which an added amount of orthoboric acid is 1equivalent was not sharp in the peak of XRD pattern, so that it isthought that a thin film in which the amorphous TiO₂ and the anatasestructure TiO₂ are mixed was formed. The result is shown as 2 in FIG. 1.

Example 5

1.75 g of (NH₄)₂ TiF₆ was added to 700 ml of purified water, and wasdissolved by stirring at 30° C. for 24 hours to obtain an aqueoussolution of (NH₄)₂ TiF₆ with a concentration of 1.263×10⁻² mol/L. To thesolution was added 17.5 g of the rutile structure TiO₂ powder, and themixture was further stirred for 24 hours. Then, the powder TiO₂ remainedwithout dissolution was separated by filtration by using a filter paperhaving pores sized 1 μm and the TiO₂ fine particles passed through thefilter paper were used as the seed crystals. Each 30 ml portion of thismixture was taken in 4 polystyrene containers. In each container wassoaked as a substrate a glass plate that had been previously subjectedto ultrasonic washing in ethanol. And an orthoboric acid aqueoussolution with a concentration of 0.5 mol/L was added in an amount ofeach 1.14 ml (1 equivalent) to the two mixtures and each 22.8 ml (2equivalents) to the rest two mixtures. The containers were thenmaintained at 30° C. for 3 days or 6 days.

The substrates thus obtained were analyzed by SEM, EDX and XRD in thesimilar manner as in Example 4. As the results, it could be confirmedthat the longer the soaking period and the more the amount of orthoboricacid added, the more precipitated amount is obtained, and that a denseTiO₂ thin film was formed on the surface of the substrate that had beensoaked for 6 days after adding 2 equivalents of orthoboric acid. For thesubstrate soaked for 6 days the formation of the TiO₂ crystals wasobserved in a similar tendency to that in Example 4, and the crystals ofthe thin film obtained was the anatase structure even though the seedcrystals were the rutile structure.

Example 6

An experiment was conducted similar to that in Example 5 except formaking the seed crystal the anatase structure TiO₂. As the results, inthe similar manner as in Examples 4 and 5, an almost dense or acompletely dense TiO₂ thin film was formed on the surface of thesubstrate that had been soaked for 6 days with 1 equivalent or 2equivalents of orthoboric acid added. Either of the TiO₂ thin film thusobtained was amorphous.

Example 7

0.231 g of (NH₄)₂ TiF₆ was added to 400 ml of purified water, and wasdissolved under stirring at 30° C. for 24 hours to obtain an aqueoussolution of (NH₄)₂ TiF₆ with a concentration of 2.918×10⁻³ mol/L. Each30 ml portion of this solution was taken in three polystyrenecontainers. In each container was added as a substrate a glass platethat had been previously subjected to ultrasonic washing in ethanol. Andan orthoboric acid aqueous solution with a concentration of 0.25 mol/Lwas added in amounts of 0.42 ml (0.8 equivalent) to the one, 0.53 ml (1equivalent) to the another one and 1.05 ml (2 equivalents) to the restone, respectively, and the containers were then maintained at 30° C. for6 days.

The substrates were taken out from the solution, washed with distilledwater and air dried. When each substrate was observed by SEM, formationof a dense thin film was observed. As the result of an XRD measurement,XRD peaks corresponding to those of the anatase structure TiO₂ powderwas observed for each substrate. Thereby it was confirmed that a thinfilm containing the anatase structure TiO₂ crystals was formed. The peakintensity was greater for the film on the substrate soaked in thesolution to which a greater amount of orthoboric acid was added.

Example 8

A thin film of the anatase structure TiO₂ crystal with a thickness of0.4 μm was formed on the inside of the glass tube with a length of 350mm and an inner diameter of 1 mm by the processesimilar to Example 1.12,000 of these tubes were bundled, and on both ends of the bundle wereinstalled circular fluorescent tubes whose shape and size were matchedto those of the above bundle of glass tubes to make a reactor for thephotocatalytic reaction. From the fluorescent tubes on both ends of thereactor, light was guided into the inside of the glass tubesconstituting the reactor.

While the reactor was being illuminated with a light, a raw water havinga BOD value of 180 mg/L polluted with organic materials was flown fromone end of the reactor at a flow rate of 2.5 L/h. The organic materialsin the water which came out from the other end were analyzed and wereexamined for the presence of odor and microorganisms. The BOD value wasfound to be 1 mg/L or less, and the analytical results and the resultsof deodorization and sterilization were sufficient and satisfactory.

Example 9

To 500 ml of purified water was added 1.0 g of (NH₄)₂ TiF₆, and wasdissolved under stirring at 30° C. for 24 hours to obtain an aqueoussolution of (NH₄)₂ TiF₆ with a concentration of 1.01×10⁻² mol/L. To thissolution was added 17.5 g of the anatase structure TiO₂ powder, whichwas further stirred. Then, the powder TiO₂ was separated by filterationby using a filter paper having pores sized 1 μm and the TiO₂ fineparticles that passed through the filter paper were used as the seedcrystals. This mixture was taken in a polystyrene container. In thecontainer was soaked as a substrate a glass plate (alkali-free glass)that had been previously subjected to ultrasonic washing in ethanol, andthen 10 g of boron oxide (B₂ O₃) was added thereto. The container wasthen maintained at 34° C. for 5 hours.

The substrate thus obtained was analyzed by the SEM, EDX and XRD in thesimilar manner as in Example 4, and as the results, it was confirmedthat a transparent anatase structure TiO₂ thin film was formed.

Example 10

To 350 ml of purified water was added 1.5 g of (NH₄)₂ TiF₆, and wasdissolved under stirring at 30° C. for 24 hours to obtain an aqueoussolution of (NH₄)₂ TiF₆ with a concentration of 2.17×10⁻² mol/L. To thissolution was added 17.5 g of the anatase structure TiO₂ powder and wasfurther stirred. Then, the powder TiO₂ was separated by filtration byusing a filter paper having pores sized 1 μm and the TiO₂ fine particlesthat passed through the filter paper were used as the seed crystals.This mixture was taken in a polystyrene container. In the container wassoaked as a substrate a glass plate (alkali-free glass) that had beenpreviously subjected to ultrasonic washing in ethanol, and then each 5 gof boron oxide (B₂ O₃) was added for 4 times in an interval of 30minutes. The container was then maintained at 35° C. for 5 hours.

The substrate thus obtained was analyzed by the SEM, EDX and XRD in thesimilar manner as in Example 4, and as the results, it could beconfirmed that a transparent anatase structure TiO₂ thin film wasformed.

Example 11

To 350 ml of purified water was added 2.0 g of (NH₄)₂ TiF₆, and wasdissolved under stirring at 30° C. for 24 hours to obtain an aqueoussolution of (NH₄)₂ TiF₆ with a concentration of 2.89×10⁻² mol/L. While17.5 g of the anatase structure TiO₂ powder was added to 400 ml ofpurified water and was stirred, then, allowed to stand for 2 days. 2 mlof the supernatant (top layer of the mixture) was added to the aboveaqueous solution to supply seed crystals. This mixture was taken in apolystyrene container. In the container was soaked as a substrate aglass plate (alkali-free glass) that had been previously subjected toultrasonic washing in ethanol, and then 10 g of boron oxide (B₂ O₃) wasadded thereto, and the container was then maintained at 35° C. for 5hours.

The substrate thus obtained was analyzed by the SEM, EDX and XRD in thesimilar manner as in Example 4, and as the results, it could beconfirmed that a transparent anatase structure TiO₂ thin film wasformed.

Example 12

To 350 ml of purified water was added 2.5 g of (NH₄)₂ TiF₆, and wasdissolved under stirring at 30° C. for 24 hours to obtain an aqueoussolution of (NH₄)₂ TiF₆ with a concentration of 3.61×10⁻² mol/L. To thissolution was added 17.5 g of the anatase structure TiO₂ powder and wasstirred. Then, the powder TiO₂ was separated by filtration by using afilter paper having pores sized 1 μm and the TiO₂ fine particles passedthrough the filter paper were used as the seed crystals. This mixturewas taken in a polystyrene container. In the container was soaked as asubstrate a glass plate (soda lime glass) that had been previouslysubjected to ultrasonic washing in ethanol, and then 10 g of boron oxide(B₂ O₃) was added thereto. The container was then maintained at 40° C.for 7 hours.

The substrate thus obtained was analyzed by the SEM, EDX and XRD in thesimilar manner as in Example 4, and as the results, it could beconfirmed that a transparent anatase structure TiO₂ thin film wasformed.

Comparative Example 1

To 350 ml of purified water was added 6.3 g of (NH₄)₂ TiF₆, and wasdissolved under stirring at 30° C. for 24 hours to obtain an aqueoussolution of (NH₄)₂ TiF₆ with a concentration of 9.10×10⁻² mol/L. To thissolution was added 17.5 g of the anatase structure TiO₂ powder and wasstirred. Then, the powder TiO₂ was separated by filtration by using afilter paper having pores sized 1 μm and the TiO₂ fine particles thatpassed through the filter paper were used as the seed crystals. Thismixture was taken in a polystyrene container. In the container wassoaked as a substrate a glass plate (alkali-free glass) that had beenpreviously subjected to ultrasonic washing in ethanol, and 10 g of boronoxide (B₂ O₃) was added. The container was then maintained at 35° C. for5 hours.

The substrate thus obtained was analyzed by the SEM, EDX and XRD in thesimilar manner as in Example 4, and as the results, it could beconfirmed that a turbid anatase structure TiO₂ thin film was formed.

Example 13

In the similar process as described in the above-mentioned Example 11, atitanium oxide film was formed on the both surfaces of a soda lime glasswith 50 mm×70 mm×1 mm in size. Each film thickness was about 0.25 μm.

To the surface of the titanium oxide film was coated 1 mg of a salad oilwith a ratio of 0.03 mg/cm², and an ultraviolet ray was irradiated by a10 W black light, then, the results as shown in FIG. 2 were obtained.

As shown in FIG. 2, the salad oil showed remarkable weight decrease sothat it could be confirmed that the titanium oxide film of the presentinvention decomposes the salad oil by the photocatalytic reaction.

We claim:
 1. A process for producing a titanium oxide film containingcrystalline titanium oxide, which comprises forming the titanium oxidefilm on the surface of a substrate in an aqueous solution containing atleast one titanium fluoro complex compound represented by the formula(I):

    A.sub.a Ti.sub.b F.sub.c                                   (I)

wherein a plural number of A are the same or different from each otherand each A represents a hydrogen atom, an alkali metal atom, an ammoniumgroup or coordinated water; and a, b and c are each a number such thatsaid complex compound is electrically neutral, and a seed crystal forproducing the titanium oxide film, in the presence of a fluorideion-capturing agent to form a titanium oxide film on the surface of thesubstrate.
 2. The producing process according to claim 1, wherein a partor whole portion of the titanium fluoro complex compound is obtained byreacting titanium oxide with hydrofluoric acid.
 3. The producing processaccording to claim 1, wherein A is selected from the group consisting ofa hydrogen atom; lithium, sodium, potassium, rubidium, cesium; anammonium group and coordinated water.
 4. The producing process accordingto claim 1, wherein the compound represented by the formula (I) is

    A.sub.2 TiF.sub.6

wherein A is as defined in claim
 1. 5. The producing process accordingto claim 1, wherein the compound represented by the formula (I) is atleast one compound selected from the group consisting of H₂ TiF₆, (NH₄)₂TiF₆, Na₂ TiF₆, K₂ TiF₆, Rb₂ TiF₆ and Cs₂ TiF₆.
 6. The producing processaccording to claim 1,wherein the titanium fluoro complex compound iscontained in an amount of 10⁻⁹ to 9×10⁻² mol/L.
 7. The producing processaccording to claim 1,wherein the titanium fluoro complex compound iscontained in an amount of 10⁻⁶ to 6×10⁻² mol/L.
 8. The producing processaccording to claim 1,wherein the titanium fluoro complex compound iscontained in an amount of 10⁻² to 4×10⁻² mol/L.
 9. The producing processaccording to claim 1, wherein the fluoride ion-capturing agent is ahomogeneous structure fluoride ion-capturing agent or a heterogeneousstructure fluoride ion-capturing agent.
 10. The producing processaccording to claim 9, wherein the homogeneous structure fluorideion-capturing agent is selected from the group consisting of a boroncompound, aluminum chloride, sodium hydroxide and aqueous ammonia. 11.The producing process according to claim 10, wherein the boron compoundis at least one compound selected from the group consisting oforthoboric acid, metaboric acid and boron oxide.
 12. The producingprocess according to claim 10, wherein the homogeneous structurefluoride ion-capturing agent is added in an amount of 10⁻⁴ to 3,000%based on an amount corresponding to an equivalent amount of fluorideions in the solution.
 13. The producing process according to claim 1,wherein the seed crystal for producing the film is a seed crystal oftitanium oxide.
 14. The producing process according to claim 13, whereinthe seed crystal is a crystal of an anatase structure titanium oxide ora rutile structure titanium oxide.
 15. The producing process accordingto claim 13, wherein the seed crystal for producing the titanium oxidehas an average particle size of 0.001 to 10 μm.
 16. The producingprocess according to claim 14, wherein the seed crystal for producingthe titanium oxide has an average particle size of 0.001 to 10 μm.
 17. Aprocess for preparing a photocatalyst comprising forming a titaniumoxide film on the surface of a substrate by the producing processaccording to claim
 1. 18. The process for preparing a photocatalystaccording to claim 17, wherein the titanium oxide has an anatasestructure.
 19. A process for producing a titanium oxide film having aphotocatalytic activity, which comprises forming the titanium oxide filmon the surface of a substrate in an aqueous solution containing at leastone titanium fluoro complex compound represented by the formula (I):

    A.sub.a Ti.sub.b F.sub.c                                   (I)

wherein a plural number of A are the same or different from each otherand each A represents a hydrogen atom, an alkali metal atom, an ammoniumgroup or coordinated water; and a, b and c are each a number such thatsaid complex compound is electrically neutral, and a seed crystal forproducing the titanium oxide film in the presence of a fluorideion-capturing agent to form a titanium oxide film on the surface of thesubstrate.
 20. The producing process according to claim 19, wherein apart or whole portion of the titanium fluoro complex compound isobtained by reacting titanium oxide with hydrofluoric acid.
 21. Theproducing process according to claim 19, wherein A is selected from thegroup consisting of a hydrogen atom; lithium, sodium, potassium,rubidium, cesium; an ammonium group and coordinated water.
 22. Theproducing process according to claim 19, wherein the compoundrepresented by the formula (I) is A₂ TiF₆, wherein A is as defined inclaim
 19. 23. The producing process according to claim 19, wherein thecompound represented by the formula (I) is at least one compoundselected from the group consisting of H₂ TiF₆, (NH₄)₂ TiF₆, Na₂ TiF₆, K₂TiF₆, Rb₂ TiF₆ and Cs₂ TiF₆.
 24. The producing process according toclaim 19, wherein the titanium fluoro complex compound is contained inan amount of 10⁻⁹ to 9×10⁻² mol/L.
 25. The producing process accordingto claim 19, wherein the titanium fluoro complex compound is containedin an amount of 10⁻⁶ to 6×10⁻² mol/L.
 26. The producing processaccording to claim 19, wherein the titanium fluoro complex compound iscontained in an amount of 10⁻² to 4×10⁻² mol/L.
 27. The producingprocess according to claim 19, wherein the fluoride ion-capturing agentis a homogeneous structure fluoride ion-capturing agent or aheterogeneous structure fluoride ion-capturing agent.
 28. The producingprocess according to claim 27, wherein the homogeneous structurefluoride ion-capturing agent is selected from the group consisting of aboron compound, aluminum chloride, sodium hydroxide and aqueous ammonia.29. The producing process according to claim 28, wherein the boroncompound is at least one compound selected from the group consisting oforthoboric acid, metaboric acid and boron oxide.
 30. The producingprocess according to claim 28, wherein the homogeneous structurefluoride ion-capturing agent is added in an amount of 10⁻⁴ to 3,000%based on an amount corresponding to an equivalent amount of fluorideions in the solution.
 31. The producing process according to claim 19,wherein the seed crystal for producing the film is a seed crystal oftitanium oxide.
 32. The producing process according to claim 31, whereinthe seed crystal is a crystal of an anatase structure titanium oxide ora rutile structure titanium oxide.
 33. The producing process accordingto claim 31, wherein the seed crystal for producing the titanium oxidehas an average particle size of 0.001 to 10 μm.
 34. The producingprocess according to claim 32, wherein the seed crystal for producingthe titanium oxide has an average particle size of 0.001 to 10 μm.
 35. Aprocess for preparing a photocatalyst comprising forming a titaniumoxide film on the surface of a substrate by the producing processaccording to claim
 19. 36. The process for preparing a photocatalystaccording to claim 35, wherein the titanium oxide has an anatasestructure.
 37. A process for producing a titanium oxide film containingcrystalline titanium oxide, which comprises forming the titanium oxidefilm on the surface of a substrate in an aqueous solution containing atitanium fluoro complex ion of the formula: TiF₆ ²⁻, by adding afluoride ion-capturing agent to said aqueous solution,wherein saidaqueous solution further contains a seed crystal for producing thetitanium oxide film.
 38. The producing process according to claim 37,wherein a part or whole portion of the titanium fluoro complex ion isobtained by reacting titanium oxide with hydrofluoric acid.
 39. Theproducing process according to claim 37, wherein the titanium fluorocomplex ion is obtained by dissolving a titanium fluoro complex compoundrepresented by the formula (I'):

    A.sub.2 TiF.sub.6                                          I(')

wherein a plural number of A are the same or different from each otherand each A represents a hydrogen atom, an alkaline metal atom, anammonium group or coordinated water.
 40. The producing process accordingto claim 39, wherein the compound represented by the formula (I') is atleast one compound selected from the group consisting of H₂ TiF₆, (NH₄)₂TiF₆, Na₂ TiF₆, K₂ TiF₆, Rb₂ TiF₆ and Cs₂ TiF₆.
 41. The producingprocess according to claim 39, wherein the titanium fluoro complexcompound is contained in an amount of 10⁻⁹ to 9×10⁻² mol/L.
 42. Theproducing process according to claim 39, wherein the titanium fluorocomplex compound is contained in an amount of 10⁻⁶ to 6×10⁻² mol/L. 43.The producing process according to claim 39, wherein the titanium fluorocomplex compound is contained in an amount of 10⁻² to 4×10⁻² mol/L. 44.The producing process according to claim 37, wherein the fluorideion-capturing agent is a homogeneous structure fluoride ion-capturingagent or a heterogeneous structure fluoride ion-capturing agent.
 45. Theproducing process according to claim 44, wherein the homogeneousstructure fluoride ion-capturing agent is selected from the groupconsisting of a boron compound, aluminum chloride, sodium hydroxide andaqueous ammonia.
 46. The producing process according to claim 45,wherein the boron compound is at least one compound selected from thegroup consisting of orthoboric acid, metaboric acid and boron oxide. 47.The producing process according to claim 45, wherein the homogeneousstructure fluoride ion-capturing agent is added in an amount of 10⁻⁴ to3,000% based on an amount corresponding to an equivalent amount offluoride ions in the solution.
 48. The producing process according toclaim 37, wherein the seed crystal for producing the film is a seedcrystal of titanium oxide.
 49. The producing process according to claim48, wherein the seed crystal is a crystal of an anatase structuretitanium oxide or a rutile structure titanium oxide.
 50. The producingprocess according to claim 48, wherein the seed crystal for producingthe titanium oxide has an average particle size of 0.001 to 10 μm. 51.The producing process according to claim 49, wherein the seed crystalfor producing the titanium oxide has an average particle size of 0.001to 10 μm.
 52. A process for preparing a photocatalyst comprising forminga titanium oxide film on the surface of a substrate by the producingprocess according to claim
 37. 53. The process for preparing aphotocatalyst according to claim 52, wherein the titanium oxide has ananatase structure.
 54. A process for producing a titanium oxide filmhaving a photocatalytic activity, which comprises forming the titaniumoxide film on the surface of a substrate in an aqueous solutioncontaining a titanium fluoro complex ion of the formula: TiF₆ ²⁻, byadding a fluoride ion-capturing agent to said aqueous solution,whereinsaid aqueous solution further contains a seed crystal for producing thetitanium oxide film.
 55. The producing process according to claim 54,wherein a part or whole portion of the titanium fluoro complex ion isobtained by reacting titanium oxide with hydrofluoric acid.
 56. Theproducing process according to claim 54, wherein the titanium fluorocomplex ion is obtained by dissolving a titanium fluoro complex compoundrepresented by the formula (I'):

    A.sub.2 TiF.sub.6                                          I(')

wherein a plural number of A are the same or different from each otherand each A represents a hydrogen atom, an alkaline metal atom, anammonium group or coordinated water.
 57. The producing process accordingto claim 56, wherein the compound represented by the formula (I') is atleast one compound selected from the group consisting of H₂ TiF₆, (NH₄)₂TiF₆, Na₂ TiF₆, K₂ TiF₆, Rb₂ TiF₆ and Cs₂ TiF₆.
 58. The producingprocess according to claim 56, wherein the titanium fluoro complexcompound is contained in an amount of 10⁻⁹ to 9×10⁻² mol/L.
 59. Theproducing process according to claim 56, wherein the titanium fluorocomplex compound is contained in an amount of 10⁻⁶ to 6×10⁻² mol/L. 60.The producing process according to claim 56, wherein the titanium fluorocomplex compound is contained in an amount of 10⁻² to 4×10⁻² mol/L. 61.The producing process according to claim 54, wherein the fluorideion-capturing agent is a homogeneous structure fluoride ion-capturingagent or a heterogeneous structure fluoride ion-capturing agent.
 62. Theproducing process according to claim 61, wherein the homogeneousstructure fluoride ion-capturing agent is selected from the groupconsisting of a boron compound, aluminum chloride, sodium hydroxide andaqueous ammonia.
 63. The producing process according to claim 62,wherein the boron compound is at least one compound selected from thegroup consisting of orthoboric acid, metaboric acid and boron oxide. 64.The producing process according to claim 62, wherein the homogeneousstructure fluoride ion-capturing agent is added in an amount of 10⁻⁴ to3,000% based on an amount corresponding to an equivalent amount offluoride ions in the solution.
 65. The producing process according toclaim 54, wherein the seed crystal for producing the film is a seedcrystal of titanium oxide.
 66. The producing process according to claim65, wherein the seed crystal is a crystal of an anatase structuretitanium oxide or a rutile structure titanium oxide.
 67. The producingprocess according to claim 65, wherein the seed crystal for producingthe titanium oxide has an average particle size of 0.001 to 10 μm. 68.The producing process according to claim 66, wherein the seed crystalfor producing the titanium oxide has an average particle size of 0.001to 10 μm.
 69. A process for preparing a photocatalyst comprising forminga titanium oxide film on the surface of a substrate by the producingprocess according to claim
 54. 70. The process for preparing aphotocatalyst according to claim 69, wherein the titanium oxide has ananatase structure.